The surface navigation of the RNAV type allows an aircraft to fly from a waypoint to another waypoint, and no longer from ground stations (of radio-navigation devices of the NAVAID type) to ground stations.
It is known that the RNP concept corresponds to a surface navigation, for which there are added (on board the aircraft) monitoring and warning devices allowing to ensure that the aircraft remains in a so-called RNP corridor, around a reference trajectory and authorizing taking into consideration curved trajectories. Outside this corridor, there is potentially some relief or other aircrafts. The required performance for a RNP operation type is defined by a RNP value accounting for half the width (in nautical miles: NM) of the corridor around the reference trajectory, where the aircraft should remain 95% of the time during the operation. A second corridor (around the reference trajectory) having half its width twice the RNP value is also defined. The probability that the aircraft goes out of this second corridor should be lower than 10.sup.−7 per hour of flight.
The concept of RNP AR operations is even more stringent. The RNP AR procedures are, indeed characterized by:                RNP values:                    being lower than or equal to 0.3 NM in approach, and able to go down to 0.1 NM; and            being strictly lower than 1 NM at start and upon re-throttling and also able to go down to 0.1 NM;                        a final approach segment that could be curved; and        obstacles (mountains, traffic . . . ) that could be located at twice the RNP value with respect to the reference trajectory, while for usual RNP operations, an additional margin with respect to obstacles is provided.        
The air authorities have defined a Target Level of Safety (“TLS”) of 10−7 per operation, whatever the type. In the case of RNP AR operations, as the RNP values could go down to 0.1 NM and the obstacles could be located at twice the RNP value of the reference trajectory, this objective results in a probability that the aircraft goes out of the corridor with half a width D=2.RNP that should not exceed 10−7 per procedure.
The equipment embedded on board aircrafts (flight management system, inertial unit, means for updating GPS data and means for guiding the autopilot), as well as the usual architecture, does not allow to reach the target level of safety, if mitigation operational means are not provided, including for the detection and the management of possible breakdowns. This is why a special authorization is required for this type of operation, so as to ensure that the operational procedures and the training of pilots allow to reach the target level of safety. Moreover, as the crew should manage some breakdowns, aircrafts are not able to-day to guarantee a RNP value of 0.1 NM upon a breakdown, as the crew are not able to meet the performance requirements in the autopilot mode.
On current aircrafts, monitoring RNP AR operations is implemented via two usual functions, that is:                a first function monitoring the accuracy and the integrity of the position calculation:                    the accuracy of the position is compared to once the RNP value;            the integrity is compared to twice the RNP value; and            if one of the two parameters, accuracy or integrity, exceeds the allotted threshold, a warning is emitted and the crew should take appropriate actions; and                        a second function allowing the crew to monitor the guidance of the aircraft:                    lateral and vertical deviations of the aircraft with respect to the reference trajectory are displayed and presented to the crew;            the crew monitors the deviations with respect to the budgets allotted for each deflection. If the crew detect an excessive deviation, they should manage the aircraft and initiate adequate correcting actions.                        
As set forth previously, current aircrafts are not able to guarantee a RNP value of 0.1 NM upon a breakdown and the crew should be specially trained for flying RNP AR procedures. The crew should, indeed, be able to, adequately, detect and process, the breakdowns being able to compromise the ongoing operation.
The objective for future aircrafts involves having the ability to fly RNP AR procedures with RNP values up to 0.1 NM, and this without restriction (in a normal situation and in the case of a breakdown), in a starting, approach and throttling up mode. To this end, the crew should no longer be primarily relied upon for detecting and processing breakdowns.
Now, a flight management assembly, of the Flight Management System (“FMS”) type, being responsible for managing the flight plan, the calculation of the trajectory and of deviations/guidance orders nominally operates with two flight management systems, operating in dual mode. In this cooperative nominal operating mode, one of the flight management systems is considered as the master, and it imposes to the other, being considered as the slave, to implement some operations in particular at the level of the management of the flight plan so as to allow the slope system to remain synchronized to said master system. Among these operations, are included, in particular, requests for segmenting the trajectory so that the two flight management systems remain synchronized and are able to provide consistent information to the crew.
Now, in a context of air operations requiring navigation and guidance performance, the lateral trajectory is a critical data, as it has a direct impact on the guidance of the aircraft and on the keeping of the protective corridor with respect to obstacles. Via the dual mode operating, an erroneous piece of information from the system could corrupt the slave system without any problem being detected, neither by the systems, nor by the crew. If an erroneous piece of information leads the master system to sequence its lateral flight plan when that is unnecessary, the master system will impose such sequencing to the slave system. Thus, the erroneous sequencing will be carried out consistently and nearly simultaneously by the two flight management systems.
Such an operating mode is therefore not compatible with carrying out air operations requiring navigation and guidance performance.